The present invention relates to receiver/decoders generally and, more particularly, to a receiver/decoder for use with otherwise incompatible systems.
The Society of Motion Picture and Television Engineers (SMPTE) has created a serial interface standard (the SMPTE-259M) for sending non-compressed component and composite digital video streams between various pieces of studio grade equipment. There is a large installed base of SMPTE-259M compliant equipment. The interface uses a phase independent form of signaling called NRZI (Non-Return-To-Zero, Invert on ones). The interface may present both true and complement data. One common form of SMPTE equipment is a serial switch that is used to route these video streams between other pieces of equipment.
Another common serial interface, the DVB-ASI (Digital Video Broadcastxe2x80x94Asynchronous Serial Interface) also makes use of a serial interface to route MPEG2 encoded video streams between pieces of equipment. The DVB-ASI interface makes use of an 8B/10B encoded data stream that, unlike SMPTE-259M, is sensitive to the phase of the data stream. Other than this one characteristic, both the DVB-ASI and SMPTE-259M physical interfaces are essentially identical. Both operate at or handle 270 Mbaud serial data, use 75 Ohm studio grade coax (e.g., Belden Type 8281), have the same launch amplitude (800 mV), connectors, etc. The physical similarity between the interfaces has led some video production studios to use standard SMPTE serial switches to route DVB-ASI data streams. Unfortunately, the configuration of such a system requires significant trial and error in connections (for proper operation) to find out which inputs/outputs do not introduce an inversion in the data stream.
FIG. 1 illustrates a conventional approach for interconnecting SMPTE and DVB devices. FIG. 1 comprises a number of SMPTE transmit devices 12a, 12b and 12n, a number of DVB transmit devices 14a, 14b and 14n, a number of SMPTE receive devices 16a, 16b and 16n, a number of DVB receive devices 18a, 18b and 18n, a SMPTE switch 20, a SMPTE switch 22 and a DVB switch 24. Since the SMPTE transmit devices 12a-12n and the SMPTE receive devices 16a-16n are base and dependent devices, the SMPTE switches 20 and 22 may present signals at either the true (+) or complement (xe2x88x92) outputs. However, the DVB transmit devices 14a-14n and the DVB receive devices 18a-18n are phase dependent devices. When connected through the DVB switch 24, a true phase is maintained between the DVB transmit devices 14a-14n and the DVB receive devices 18a-18n. The DVB transmit device 14a is shown connected through the SMPTE switch 22. In such a configuration, it is important to maintain the positive phase at the output of the SMPTE switch 22 prior to presenting it to the DVB receive device 18b. Conventional approaches require the implementation of either DVB-ASI specific equipment, or identification of ports on SMPTE equipment that do not introduce an inversion in the data stream.
Referring to FIG. 2, a block diagram of a conventional circuit 30 is shown. The circuit 30 generally comprises data cable 32, a receive equalization circuit 34 and a logic circuit 36. A logic circuit 36 generally presents a framed and decoded data signal at an output 38 and an error detected signal RVS at an output 40. The logic 36 generally comprises a differential amplifier 42 and a decoder framer 44. The signal received by the circuit 30 must have a proper phase relationship in order for the decoder framer 44 to provide a proper functioning at the output 38. See, for example, U.S. patent application Ser. No. 08/658,760, now U.S. Pat. No. 5,754,011 filed Jun. 5, 1996, the appropriate sections which are hereby incorporated by reference.
The existing methods all have significant drawbacks for video studios implementing DVB-ASI. Such studios must either abandon their existing SMPTE switches when they add on DVB-ASI compliant equipment, duplicate and segregate their existing switches into SMPTE and DVB cable plants, or reduce the usable port capacity of their SMPTE equipment by a factor of two. The costs and logistics of any of these techniques could significantly slow the adoption of DVB-ASI.
The present invention concerns a circuit comprising a receiver configured to receive a first signal having a first phase, a second signal having a second phase opposite the first phase and an output configured to present either the first or second signals. A state machine may be configured to receive the output of the receiver circuit and to provide a control signal configured to select the first or second signal.
The objects, features and advantages of the present invention include providing a phase independent transmit and/or receive device that may be implemented using switches that may invert the polarity of the signal and may independently and/or automatically compensates for phase inversions.